1. Field of the Invention
This invention relates in general to the technology of ion and plasma sources, and more particularly to a Hall-current ion source designed for producing broad ion beams of various energies utilized in thin film technology and material processing.
2. Description of the Prior Art
A Hall-current ion source, in some cases also called end-Hall ion source, was described in U.S. Pat. No. 4,862,032 by Kaufman et al. Later it was modified in U.S. Pat. No. 6,608,431 by Kaufman as an ion source of a modular design for easy assembly/disassembly. A very similar concept of a Hall-current ion source was described in a U.S. Pat. No. 6,645,301 by Sainty. Varieties of another Hall-current ion source type called as closed drift ion sources in a form of electric propulsion thrusters and ion sources are known and described by Zhurin et al in article “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1. A hybrid ion source of end-Hall type and closed drift ion source is presented in U.S. Pat. No. 7,116,054 by Zhurin.
This ion source utilizes features of both types of ion sources and provides more efficient ionization and acceleration of ion beam than regular end-Hall ion source. In this ion source, in comparison with Kaufman et al. and Sainty's end-Hall ion sources having magnetic field with reducing its strength from a gas distributing system to a discharge channel exit, there is utilized a positive gradient of magnetic field in a discharge channel for efficient acceleration of ions and for suppression of high amplitude discharge current and voltage oscillations. These publications are incorporated herein by reference. A Hall-current ion source belongs to a family of gridless ion sources and was introduced together with a gridded ion source for industrial applications. Description of gridded ion source for technological applications was given by Kaufman in article “Technology of Ion Beam Sources Used in Sputtering”, in Journal of Vacuum Science & Technology, Vol. 15, pp 272-276, March/April 1978. This publication is also incorporated herein by reference. All these ion sources are spin-offs from electric propulsion thrusters utilized with space satellites for producing thrust to move satellite with a certain momentum to a designated position in space.
If gridded ion sources can be considered as electrostatic ion sources, then gridless ion sources can be called electromagnetic ion sources. Operation of all Hall-current ion sources is based on electrical discharge in gas in magnetic field at pressures of about 10−5-10−3 Torr with reduced mobility of electrons in direction across to magnetic field lines. Because of this, it becomes possible in a direction of magnetic field lines to develop quite strong electric field strength that provides acceleration of ions.
In order to maintain electrical discharge in gas it is necessary to utilize conditions, which include presence of charged particles that depend on magnetic field value and its geometry, shape of electrodes and other factors influencing charge transportation in plasma. It can lead to a separation of charge particles caused by different trajectories and velocities of ions and electrons. Such separation generates a Hall current, which is directed along a normal to vectors of discharge current, I and magnetic field, B.
Hall current plays a major role in ion acceleration in plasma when a characteristic time of a process is an order of a period of charged particles rotation along a Larmour radius τ≧1/ωi with a condition that an electron component becomes “magnetized”. “Magnetized” plasma means that both electron and ion components of plasma experience many revolutions around a magnetic field line before they move due to a collision with a neighbor particle. However, it is not necessary that both plasma components will be magnetized. In the case of ion sources, only electrons are usually “magnetized” and ions are not magnetized.
It is important that for a Hall-current ion source there will be performed the following condition: rLe<<l<<rLi, where l is a characteristic dimension of acceleration zone of a discharge region (length or width of a discharge channel), rLe and rLi are Larmour radii, correspondingly for electrons and ions.
Electrons drift along equipotential surfaces, which are presented by magnetic field lines, and ions are accelerated in a direction of electrical field practically without any influence of magnetic field, because they are not magnetized as electrons. Electrons in existing Hall-current ion sources invented by Kaufman et al. and Sainty have strong axial magnetic field component, Bz in area of a gas distributing system and a hollow anode bottom part, and only close to exit from a discharge chamber magnetic field lines acquire a substantial value of radial magnetic field component, Br. The features of low value of radial component of magnetic field in a gas distributing system and anode area of Hall-current ion sources and influence of axial component on an ion beam current value were discussed in detail by Zhurin et al. in the above mentioned article “Physics of Closed Drift Thrusters” in Plasma Sources Science & Technology, Vol. 8 (1999), beginning on page R1 and by Zhurin in US 2005/0237000 A1.
Regular Hall-current ion sources, in some cases called end-Hall ion sources, fabricated by Veeco Instruments through a license from Kaufman & Robinson Inc., by Kaufman & Robinson Inc. and Saintech Ltd operate with working reactive gases such as Oxygen, Nitrogen, and noble gases such as Argon, Xenon and other gases at discharge voltages, of about Vd=100-300 V (O2, N2), and Vd=80-300 V (Ar, Xe), and discharge currents from about Id=1 A up to 10-15 A. For most thin film deposition processes including a so-called ion assisted deposition and a sputtering deposition, it is necessary to have energies of ions in the range of 10-30 eV for ion assisted deposition, and in the range of 100-500 eV for a sputtering deposition. An ion beam mean energy of end-Hall ion sources in general is about 60% of discharge voltage, Vd. In other words, in order to have a range of mean ion energies from 15 to 500 eV an ion source should operate at discharge voltages from about 25 V to 830 V.
In a publication “Low-Energy End-Hall Ion Source Characterization at Millitorr Pressures” by Kahn et al., SVC (Society of Vacuum Coaters) 48th Annual Technical Conference Proceedings, p 445, 2005, there is presented an end-Hall an ion source EH-1000 with a modified anode that is designed to sustain oxidizing effects on anode performance. This publication is incorporated here by reference. This end-Hall ion source generates ion beams of low energy of about 25 eV with Argon as working gas, where discharge current is 10 A and discharge voltage is 41-43 V. In this case, the operation of ion source with an ion beam of low energies is realized by a high mass flow of working gas, so a background pressure in a vacuum chamber is about 1-2 mTorr. At such high gas pressures in a vacuum chamber a role of charge-exchange mechanisms becomes very important, in the way that ion beam particles-ions exchange energy and momentum with neutrals that become neutrals of high energy and a target receives a flow of energetic neutrals instead of ions. The charge-exchange mechanism plays especially important role at low energies of ions. Resonance charge exchange (on atoms of the same gas) cross section in such conditions is quite high: σch-ex≈0.5×10−14-10−15 cm2. During charge exchange a fast ion becomes a fast atom, and slow atom becomes a slow ion, the whole process makes good quantified reliable results for sputtering quite difficult.
Comparatively recent new technique called a biased target deposition was introduced by Zhurin et al. in article “Biased Target Deposition” in Journal of Vacuum Science & Technology, A 18(1), January/February 2000, p 37. In this article, a compact end-Hall ion source Mark-1 was used with low energy ions applied to a target at ion energy lower than a sputtering energy threshold. This publication is incorporated here by reference. Low energy ion beam interacts with a negatively biased target of several hundreds of electron volts, and ions are accelerated in a short Debye layer (typically 0.1-1 mm, depending on pressure in vacuum chamber). Such a technique is useful for obtaining very fine thin film depositions that are not contaminated by interaction of a low energy ion beam with a target surrounding, because an ion beam energy is not sufficient to sputter unwanted particles, which are not at a negative high potential. In this case, an ion beam interacts only with target having a negative potential. A Hall-current ion source of low energy ions is good candidate for a biased target deposition technique.
An ion assisted deposition is utilized as additional flow of low energy ions applied on a substrate deposited thin film that provided by a high-energy beam sputtering of a target. Low-energy ions from a secondary ion source help to improve adhesion of applied film, to control its stress, to increase hardness, density and structure, making possible to obtain a preferred crystal orientation of a deposited thin film. Recent trend in ion assisted deposition technique is a utilization of low-energy ion beam under 50 eV, so that an ion assist beam modifies a thin film deposition without its sputtering.
The range of discharge voltages of existing regular end-Hall ion sources at pressures in vacuum chamber of 10−5-104 Torr, Vd=80-300 V, is not satisfactory for obtaining low energies for certain thin film processes (lower than 50 eV, and about 15-20 eV) with ion assisted tasks and also is not sufficient to provide high sputtering or etching rates with required optimum energies (300-500 eV).
The most usable in thin film industry end-Hall ion sources such as Mark-2 (at present time, produced by Veeco Instruments) and EH-1000 (at present time, produced by Kaufman & Robinson Inc.) utilize as magnetic means permanent magnets with a magnet's strength of about 1.3-1.5 kG at a magnet's North Pole side. There are no any available publications about a role of magnetic field value and its influence on a range of existing operating discharge voltages of 80-300 V with mean ion energies of about 50-180 eV. If Mark-2 and EH-1000 operate up to 300 V, then Sainty's ST3000 operates up to 225 V.
Since the introduction of end-Hall ion source in 1989 there was not much done in improvement of operation characteristics such as broadening of discharge voltage range of end-Hall ion sources. The introduction of a water-cooled anode helped to use higher discharge currents up to 10-15 A, but with low discharge voltages, not over 150 V. In a patent U.S. Pat. No. 6,750,600 B2 “Hall-Current Ion Source” by Kaufman et al. there was introduced a grooved anode for better operation with reactive gases. This publication is incorporated herein by reference. Anode grooves help to reduce influence of oxidized substances on anode operation, because anode covered with oxide film loses its electrical conductivity and gradually increases designated discharge voltages to high undesirable values at constant discharge current, or gradually reduces discharge current with constant discharge voltage. All these conditions depend on a Power Supply, whether it operates with a constant current, or a constant voltage mode. Anode grooves have areas that will be not covered with oxide film, because these areas will be not “seeing” oxide particles that travel in straight lines from outside of an ion source.
There is nothing much changed in a working gas distributing system of existing Hall-current ion sources. Working gas is applied through holes in a gas distributor (often called reflector) under a hollow anode bottom part. The reflector is placed between anode and a permanent magnet and also serves as a shield between hot ionized plasma consisting of high energy ions and low energy electrons supplied by a cathode made of Hot Filament, or Hollow Cathode; this reflector-shield protects a permanent magnet from overheating and direct impact from plasma. An ion beam that is developed at a discharge channel is “supposed” to flow to an ion source exit, but quite a good part of an ion beam flows into opposite direction, into a reflector's surface. Reflector usually after about 20-25 hours of operation at moderate discharge currents (about 5 A and over) and regularly used “optimum discharge voltages”, Vd=100-150 V (end-Hall Mark-2 and EH-1000 have a maximum ion beam current value at about Vd=100-125 V, and Oxygen's ion beam current is higher than Argon's ion beam current by about 20% at the same discharge current and voltage) becomes sputtered in a center part and eventually eroded into quite a substantial hole, so it is necessary to substitute such reflector for a new one.
Some users of end-Hall ion sources that understand the problem of a reflector's sputtering are trying to reduce this damage with theirs own means, or to make reflector's substitution easier and convenient. One of recently approved U.S. Pat. No. 6,963,162 B1 “Gas Distributor for an Ion Source” by Centurioni describes substitution of a reflector's eroded part with an insert of about 1.8 cm in diameter that can be placed in a central part of a reflector. After a certain time of operation this insert is substituted for a new one through an ion source top with tweezers, or similar instrument. Such substitution certainly makes sense, especially if one wants to utilize a reflector's central eroded part with an expensive material such as Tantalum, or Molybdenum-Rhenium, etc. However, the manipulation presented in a Centurioni's patent does not reduce erosion of a reflector, and a reflector's erosion problem remains unsolved. A Centurioni's patent publication is incorporated herein by reference.
There are other problems with regular end-Hall ion sources such as a high level of discharge current and voltage oscillations at higher discharge voltages over, Vd≧250 V and a low efficiency of transformation of a discharge current into an ion beam current. This problem was discussed in U.S. Pat. No. 7,116,054 “High-Efficient Ion Source with Improved Magnetic Field” by Zhurin in a hybrid Hall-current ion source that has both features of end-Hall and closed drift ion sources.
There are no quantified values for magnetic field in end-Hall ion sources that could help to select the correct optimum magnetic field value for certain operation conditions and discharge voltage ranges (energies) utilized in technological processes requiring necessary values of ion beam energies and ion beam current densities.